Skrulls are shape changing aliens in the Marvel universe. It is not clear if Marvel is able to use Skrulls in the cinematic universe or if they are entirely owned by Fox through their Fantastic Four License. The Skrulls have been an integral part of story lines involving the Fantastic Four, the Avengers, and are a constant presence in Marvel's Cosmic stories. In the film Marvel Cinematic Universe the Skrulls will be the primary villains in the film Captain Marvel.

Appearance:
Skrulls are reptilian and thus look green and scaly with pointy ears. They have highly unstable genes which allows them to alter their appearance completely. They alter appearance, gender, and in special instances super powers. They can look like a lamp, or cow provided it is roughly their same physical size. Skrulls are known for studying the Heroes of Earth to duplicate their powersets.

Changing Gender for a Skrull has been described as being similar to a human changing its hair color.

Government:
Skrulls are ruled by royalty based system where an Emperor or Empress rules based off heredity. At one point their capital is annihilated by Galactus killing the Emperor and his direct line. This leaves in the Skrull empire in anarchy with numerous Governors claiming control of the empire.
The Skrull Empress Verkake manages to unite the Skrull empire but is killed by Norman Osborn during the final battle of Secret Invasion.
Skrulls in the present comic universe are weak after numerous catastrophes but are aggressively looking for ways to increase their power.

Notable Skrulls:

Kl'rt - Super Skrull has the full powers of the Fantastic Four

Lyla- Skrull spy who falls in love with the Human torch and eventually marries him before being killed.

Secret Invasion:
Marvel had a major event that pulled in all of its Titles called Secret Invasion. It would be a crime to discuss the Skrulls without mentioning Secret Invasion.
Secret Invasion is my person favorite of all of the comicbook events. The idea of Secret Invasion is that some heroes, and government officials had been replaced by Skrull imposters. Some of the imposters had been in place for years and over a multitude of comics. Bendis the writer of Secret Invasion and its mastermind had written in clues and hints of Secret Invasion into his New Avengers run. All of the preparation made for a fun, intrigue filled event where everyone debated who was a Skrull and who was not.

On top of infiltrating the Earth in various sneaky ways the Skrulls also created a huge amounts of Super Skrulls with powers similar to those of the Earth heroes. This led to a massive menagerie of fascinating Skrulls to look out for. I remember just looking at big splash pages of Skrulls trying to pick out what powersets they were using.

Also the overall promotional art and events in my opinion was beyond anything seen any event. With catch phrases like: EMBRACE CHANGE and WHO DO YOU TRUST Marvel release a variety of posters and online pictures that hinted at an entirely new universe. A universe where the Marvel Heroes have potentially lost? Skrull propoganda! It was everywhere and in my opinion AWESOME.

History:
The Skrulls were experimented on by the Celestials leaving 3 branches of Skrulls.
2 of the branches are killed off by the remaining Deviant Branch of Skrulls. The Deviant Branch were the only branch capable of changing shape. The Deviant Skrulls have unstable DNA that allows them shapeshift and downright alter themselves very quickly. They build a vast cosmic empire. They become peaceful using their shape changing abilities to look like other aliens aided them in Diplomacy. Eventually a delegation of Skrulls comes to the planet Hala.

On Hala the Skrulls find two intelligent species and do not know whom they should deal with. So they take a group of each species Kree and Cotati to a barren world, give them resources and challenge them to build something magnificent.

The Cotati build a massive garden. The Kree a magnificent city on the Moon of Earth.
The Kree learn that the Skrulls may be favoring the Cotati. Enraged the Kree devise a new way of showing their superiority. They kill of the Cotati. Horrified by the Kree the Skrull Delegation tries to leave but are also killed by the Kree. The Kree have years before the Skrulls learn what happened to the Delegation. In this time the Kree reverse engineer the Skrull ships and build their own fleet. They attack the Skrull empire leading to the first of many Skrull / Kree wars.

The Skrull home planet is further devasted when it is attacked by Galactus the World Eater. This also leaves the empire in anarchy as Governors fight with each other for control of the empire.

The Skrulls then attack Earth in a bid to return to Supremacy but are defeated. Leaving them further weakened and without Leadership.
Incredible Hulk 419Marvel Store
Toylab is a member of the Amazon Affiliate Program which allows us to bring you awesome deals directly from the Amazon like the ones below:

MCU:

Concept Art of Skrulls in Captain Marvel film

In the film Marvel Cinematic Universe the Skrulls will be the primary villains in the film Captain Marvel.
It had long been thought that Marvel could not use the Skrulls and that their rights belonged to Fox as part of the Fantastic Four license. However, Director James Gunn and Kevin Feige in a series of comments explained that specific characters may belong to Fox but the entire race of Skrulls does not. This likely means that Super Skrull belongs to Fox but the species of the Skrulls is jointly owned.